136 CORALS AND CORAL ISLANDS. 
4, Beach and Drift formations, produced by coral accu- 
mulations on the shores through the action of the sea and 
winds, 
The outer and inner reefs, channels, and beaches, act each 
their part in producing the coral formations in progress about 
islands. 
Il. OUTER REEFS. 
The barrier and other outer reefs are always submerged at 
high tide, except where elevated at surface by accumulations 
of beach sands. The level is generally that of about one third 
tide. The coral rock is built up by the agencies at work to 
this level, and hence the existence of the broad plattorm-like 
top of the barrier. The surface is however not even, for there 
are many pools of water over it, even at the lowest tides, espe- 
cially toward its outer limits, where corals of various kinds are 
crowing luxuriantly, with fit associates of shells, star-fishes, 
echini, holothurias with their large flower-bearing heads, 
sponges, corallines and sea-weeds, making scenes of rare beauty. 
The growing corals are, however, most abundant along the outer 
margin of the reef, and in the adjoming shallow seas. Here 
they grow in profusion ; but yet the eager lover of coral land- 
scapes will be often disappointed by finding among the crowd- 
ed plantations, extensive areas of coral sand. 
The outer margin of the reef receives the plunging waves, 
and under this action, and the consequent unequal growth of 
the corals, the outline is very irregular, being often deeply cut 
into, and hence having sometimes long channels that give en- 
trance to the surging tide, and to the currents that flow back 
in preparation for the next breaker. From it, seaward, the 
depth of water usually sinks off rapidly from three to six fath- 
oms, and then falls away more gradually for many rods, or it 
